The present invention relates to LCD projectors, more particularly to an optical system for an LCD projector, comprising 3 LCD panels capable of separating red, green and blue light elements of light emitted by a light source thereof, and then composing them.
A known optical system of the LCD (Liquid Crystal Display) projector of the type to which this invention pertains is shown by way of example in FIG. 1. It comprises an illuminating device 21 including a high luminance halogen lamp as a light source, an infrared filter 22 positioned on a first optical path from the halogen lamp of the illuminating device 21 and adapted for filtering out infrared rays from the light emitted by the halogen lamp. Also, the conventional LCD projector is provided with a 45.degree. inclined blue light reflection mirror 23 positioned on the first optical path from the halogen lamp in a distance after the infrared filter 22, and adapted for separating blue light element from the light being transmitted through the infrared filter 22 and for reflecting the blue light element along a first B optical path perpendicular to the first optical path. There is also a first 45.degree. inclined total reflection mirror 31 positioned on the first B optical path in order to totally reflect the B light element reflected by the B light refection mirror 23 to a second blue optical path perpendicular to the first blue optical path, or parallel with the first optical path. On the other hand, a light transmitted through the B light reflection mirror 23 is filtered by a 45.degree. inclined green light reflection mirror 24 positioned on the first optical path after the B light reflection mirror 23, and the green light element is therefrom filtered out. Thereafter, the G light element is reflected along a first green optical path perpendicular to the first optical path. In result, only red light element is transmitted through the G light reflection mirror 24, then reflected by a second 45.degree. inclined total reflection mirror 25 positioned on the first optical path after the green light reflection mirror 24. The red light element reflected along a first red optical path perpendicular to the first optical path, or parallel with the first B and G optical paths, reaches a third 45.degree. total reflection mirror 26 which totally reflects the red light element to a second red optical path perpendicular to the first R optical path. Also, the blue, green and red light elements, respectively reflected by the first total mirror 31, the G reflection mirror 24 and the third total reflection mirror 26, are respectively transmitted through three LCD panels 30, 29 and 28 in which an image reemerges in respective colors. Thereafter, the reemerging image is applied to a dichroic prism 27 in which the blue and red color images are reflected in 90.degree. reflection angle, and the green color image is transmitted, thereby causing the three color images to be composed into a desired color image. The composed color image from the prism is projected on a screen as an enlarged image by a projection lens 32.
However, there have been some problem in the above-mentioned conventional LCD projector, such as the low grade of luminance because of using the halogen lamp for a light source, and the more intense radiation of the R light element than those of other light elements. Therefore, the length of the optical path of the R light element is longer than those of the other optical paths as shown in FIG. 1 in order to compensate for the intense radiation of the R light element. But this compensation has not been sufficient and thus the projected image on the screen is almost seen in red, thereby causing the image of high resolution power to be unobtainable. On the other hand, it is difficult to manufacture the dichroic prism used in the conventional LCD projector and furthermore, the dichroic prism is a high cost, heavy and fragile element.